1. Field of the Invention
The invention relates to the use of spectroscopy to characterize living tissue. More particularly, the invention relates to an apparatus and method for quantifying tissue hydration in a living subject non-destructively, based on irradiation of the skin tissue with near infrared light energy.
2. Description of Related Art
Near infrared (NIR) tissue spectroscopy is a promising nondestructive technology that bases measurements on the irradiation of a tissue site with NIR energy in the 700-2500 nanometer wavelength range. The energy is focused onto an area of the skin and propagates according to the scattering and absorption properties of the skin tissue. Therefore, the reflected or transmitted energy that escapes and is detected provides information about the tissue volume that is encountered. Specifically, the attenuation of the light energy at each wavelength is a function of the structural properties and chemical composition of the tissue. Tissue layers, each containing a unique heterogeneous particulate distribution, affect light absorbance through scattering. Chemical components such as water, protein, fat and blood analytes absorb light approximately proportionally to their concentration through unique absorption profiles or signatures. The measurement of tissue properties, characteristics or composition is based on detecting the magnitude of light attenuation resulting from its respective scattering and/or absorption properties.
Stratum Corneum Hydration Measurement
The quantification of hydration of the stratum corneum has commercial benefits in certain industries for monitoring skin condition and for attaining a better understanding of how hydration affects the stratum corneum. The current method of measuring the hydration of the stratum corneum non-invasively is based on the electrical characteristics of the stratum corneum. The technology measures the capacitance, admittance, impedance, or susceptance of the stratum corneum.
Spectroscopic approaches to measuring hydration of the stratum corneum have been explored. See, for example, R. Potts, D. Guzek, R. Harris, J. McKie, A Noninvasive, In Vivo Technique to Quantitatively Measure Water Concentration of the Stratum Corneum Using Attenuated Total-Reflectance Infrared Spectroscopy, Archives of Dermatological Research, Springer-Verlag, vol. 277, (1985). Potts, et al. performed a variety of in vitro experiments using Attenuated Total Reflectance (ATR) spectroscopy in the infrared region of light, and determined that hydration of the skin was highly correlated (0.99) to the ambient humidity. He developed a variety of preprocessing techniques like the protein ratio and the moisture factor to measure the hydration of the stratum corneum. He concluded that water content in the stratum corneum could be measured in vitro using ATR infrared spectroscopy. The Potts teachings however are directed to an in vitro method and are therefore unsuited to noninvasive, in vivo measurements.
Martin did a series of experiments related to in vivo measurement using diffuse reflectance near infrared spectroscopy. See K. Martin, Direct Measurement of Moisture in Skin by NIR Spectroscopy, Journal of Society of Cosmetic Chemists, vol. 44 (1993). Martin""s work lead to the finding that three different types of water may be detected in the spectra of skin. The different types of water were found in the overtone region (1058-1950 nm) using the second derivative of the spectrum; second derivative intensities were found to correlate with ambient humidity levels. It was found that the bulk water of the stratum corneum correlates most directly with ambient humidity. Bulk water was water that mostly resembled that of regular water and was not bound to any protein. It was also found that the primary hydration water correlated the least with ambient humidity.
Martin""s further work investigated the use of measuring sites at a variety of body locations having skin of varying thickness. See K. Martin, In Vivo Measurements of Water in Skin by Near Infrared Reflectance, Applied Spectroscopy, vol. 52(7)(1998). While a higher standard deviation was noted, the previous correlations with different water types in the skin were confirmed. Additionally, light scattering by the skin was found to decrease with increasing hydration. The Martin teachings, however, do not address the persistent problem in the art of compensating for structural and physiological variation between individuals or variation over time within the same individual.
Analyte Estimation
While noninvasive estimation of analytes, such as glucose concentration, has been pursued through NIR spectroscopy, the reported success and product viability has been limited by the lack of a system for compensating for structural variations between individuals that produce dramatic changes in the optical properties of the tissue sample. For example, see O. Khalil, Spectroscopic and clinical aspects of non-invasive glucose measurements, Clin Chem, vol. 45, pp 165-77 (1999) or J. Roe, B. Smoller, Bloodless Glucose Measurements, Critical Reviews in Therapeutic Drug Carrier Systems, vol. 15, no. 3, pp. 199-241, 1998. These differences are largely anatomical and provide distinct systematic spectral absorbance features or patterns that can be related directly to specific characteristics such as dermal thickness, protein levels and hydration. While the absorbance features are repeatable within a subject, over a population of subjects they produce confounding nonlinear spectral variation. Therefore, differences between subjects are a significant obstacle to the noninvasive measurement of analytes through NIR spectral absorbance.
A related U.S. Patent Application, S. Malin, T. Ruchti, An intelligent system for noninvasive blood analyte prediction, U.S. patent application Ser. No. 09/359,191 (Jul. 22, 1999), now U.S. Pat. No. 6,280,381, discloses an apparatus and procedure for substantially reducing this problem by classifying subjects according to major skin tissue characteristics prior to blood analyte estimation. The selected characteristics are representative of the actual tissue volume irradiated and the amount of the target analyte that is sampled. By grouping individuals according to the similarity of spectral characteristics representing the tissue structure, the nonlinear variation described above is reduced and estimation of analytes becomes more accurate.
The present invention provides a novel apparatus and related procedures for the quantification of hydration of the tissue through NIR tissue spectroscopy having particular benefit in several areas, including tissue state evaluation and analyte estimation. The invention utilizes a spectroscopic technique such as NIR diffuse reflectance to measure the hydration of the stratum corneum. A spectroscopic apparatus in conjunction with an optical subject interface is used to measure tissue properties and characteristics non-destructively, that are manifested spectrally and vary systematically according to the hydration of the subject""s stratum corneum.
The procedure for quantifying tissue hydration, particularly the stratum corneum, involves a calibration model that is empirically derived from a set of exemplary samples consisting of NIR tissue measurements and corresponding independent measurements made with a corneometer. The model is a set of parameters and computer generated code that is implemented to predict the hydration of the subject""s stratum corneum. The general procedure involves the steps of taking spectral measurements, typically in the near IR region of 700 to 2500 nm; detecting outliers, invalid measurements resulting from poor sampling technique, or instrument problems, or a subject outside of the calibration set; preprocessing, in which the spectral measurements are subjected to various operations that attenuate noise and instrumental variation; and estimation, in which the previously mentioned calibration model is applied to arrive at an estimation of the hydration of the subject""s stratum corneum.